Increased levels of unconventional or asymmetric warfare have led to the need to protect vehicles and/or personnel from munitions typically used in this type of warfare, such as small arms fire and improvised explosive devices (IEDs). While a variety of means are available to minimize casualties from these threats, such as increased training and “render safe” procedures, the use of armor shielding remains an important last line of defense. As a result of the need to protect a large number of potential targets while not hindering their mobility, it is also important to be able to provide armor shielding that is lightweight and relatively inexpensive.
One method of providing armor that is lighter and stronger is to use composite armor. Composite armor consists of different materials such as metals, plastics, or ceramics that together provides an armor that is stronger and lighter than traditional pure metal armor. A relatively famous form of composite armor is so called “Chobham armor,” that sandwiches a layer of ceramic between two plates of steel armor, and is used on main battle tanks such as the Abrams, where it has been proven to be highly effective in defeating high explosive anti-tank (HEAT) rounds. However, while “Chobham armor” is well suited for use placement on a main battle tank, it is too heavy and expensive for use on lighter fighting vehicles or transports.
Composite materials have also been prepared for use as lightweight armor for lighter fighting vehicles. A relatively common vehicle that has been protected using lightweight composite material is the M1114 High Mobility Multi-Purpose Wheeled Vehicles (HMMWV). The composite used to armor the HMMWWV is called HJ1. This material includes high-strength S-2 Glass™ fibers (Owens Corning) and phenolic resin that complies with MIL-L-64154 requirements, and is laminated into hard armor panels that offer significant protection against fragmented ballistic threats when compared to monolithic systems on an equivalent weight basis. However, relatively simple fiber-based composite armors have difficulty protecting vehicle occupants against many common ballistic and blast threats.
Armor piercing (AP) ammunition is designed to penetrate the hardened armor of modern military vehicles. It typically includes a sharp, hardened steel or tungsten carbide penetrator covered with a guilding metal jacket that adds mass and allows the projectile to conform to a rifled barrel and spin for accuracy. When an AP round hits armor, the guilding is rapidly deformed and drops away, leaving the sharpened penetrator traveling with a high velocity to bore its way through the armor. Studies indicate that sharp-nosed projectiles tend to move the fibers within the composite laterally away from the advancing projectile, resulting in kinked fibers around the penetration cavities but with little energy absorption. Thus, the primary reason why armor-piercing projectiles are so effective against fiber-based composite armor is that neither the fiber nor matrix material of the composite is hard enough to cause deformation of the sharp, hardened penetrator nose.
Ceramic faced armor systems were thus developed to defeat AP ammunition by breaking up the projectile in the ceramic material and terminating the fragment energy in the backing plate that supports the ceramic tiles. During impact, the projectile is blunted and cracked or shattered by the hard ceramic face. Fragmentation and comminution are produced in the ceramic and the projectile, resulting in fine ceramic rubble traveling with the projectile. The incident momentum of the initial projectile is thus transferred to fragments of shattered projectile and the ceramic rubble. The ceramic rubble typically has a mass comparable to the initial projectile; hence, the final shattered projectile and ceramic rubble exhibit a much lower impact velocity on the backing plate.
Unfortunately, during this process, the armor system is typically damaged. In order for such systems to defeat additional impacts of the threat that are near to previous impacts, the size of the damaged area produced in the armor system needs to be controlled and minimized. With better damage control, the damage size produced is smaller and more closely spaced hits can be defeated by the armor. Armor systems containing segmented ceramics in the form of “tiles” solve a part of this problem because cracks cannot propagate from one tile to another. However, strong stress waves can still damage tiles adjacent to the impacted tile by propagating through the edges of the impacted tile and into adjacent tiles. Ceramic tiles can also be damaged by the deflection and vibration of the backing plate. In addition, impact from the lateral displacement of material during ceramic fracturing can crush and damage adjacent tiles. These armor damage mechanisms must be suppressed in order to provide armor with the ability to reliably defeat multiple projectile impacts.
Additional examples of attempts to provide composite armor suitable for deployment on personnel and lighter fighting vehicles are provided by U.S. Pat. No. 6,575,075 (issued to Cohen) and U.S. Pat. No. 6,912,944 (issued to Lucuta et al). These patents provide a ceramic along with a polymer to constrain the fractured ceramic in a localized area. Cohen describes a composite armor plate that includes a layer of pellets held together as a plate by a “solidifying material” (e.g., an epoxy or thermoplastic polymer) such that the pellets form a plurality of adjacent rows. The pellets are formed from glass or ceramic, and include a channel on the inward-facing side of the pellet in order to reduce its weight. Lucuta et al. describes a ceramic armor system that includes a ceramic plate formed from a plurality of interconnecting ceramic tiles. The ceramic tiles have a flat ceramic base upon which are disposed a plurality of smaller nodes, which are asserted to provide a greater degree of protection and contribute to the scattering of radar signals. In particular, nodes are formed from partial nodes at the edges of the ceramic tiles to protect the joining sites between tiles. The ceramic armor system further includes a spall layer bonded to the front surface of the ceramic plate, a shock-absorbing layer bonded to the rear surface of the ceramic plate, and a backing bonded to the rear surface of the shock-absorbing layer. The nodes however, do not cover the entire surface, i.e., a portion of the surface is flat and hence not oriented (to the direction of perceived threat) for deflection.
However, these examples do not provide guidance on how to provide composite armor that achieves an areal density well below the areal density of rolled homogeneous armor or similar steel armor solutions needed to defeat a ballistic threat. Areal density measures the ability of an armor to provide protection for a given weight, and is measured in pounds per square foot. For example, in Lucuta et al., the thickness of the ceramic tile will always be above the critical limit needed to defeat a projectile, resulting in the presence of excess material that will result in increased areal density. These forms of armor have not ensured that the tile thickness and therefore the areal density is not excessive without sacrificing ballistic performance.
There thus remains a need for composite armor that is more lightweight, inexpensive, compact, durable, or protective, or exhibits a combination of improvements in these areas.